Flex AdvancedDatagrid : toggling visibility of columns when parent is expanded and closed

Question

HI I am new to Flex and was wondering, when we use an AdvancedDataGrid, only when we click on the parent element the children details get populated in the corresponding columns, right? So now i have made the empty columns invisible(at design) now how do i make them visible at run time when the parent element is expanded..similarly once the columns are visible, how can i make them invisible again when the parent element is closed. Please help me out

Thanks